Polarization light splitting film, backlight system and liquid crystal display having particular diffusion layer under optical rotation selection layer

ABSTRACT

A polarization light splitting film having a light receiving side and a light transmitting side. The polarization light splitting film includes an optical rotation selection layer at the light receiving side for reflecting one of right and left circularly polarized components of a light beam that is incident on the light receiving side and for transmitting the other one of the right and left circular polarization components of the light beam, and a quarter-wave layer laminated over the optical rotation selection layer at the light transmitting side.

This application claims the benefit of Japanese Application No.09-48562, filed on Feb. 18, 1997, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light splitter and, moreparticularly, to a polarization light splitting film for use in a liquidcrystal display device and the like.

2. Discussion of the Related Art

Due to recent technological development, liquid crystal displays (LCDs)are becoming increasingly popular for use in the display component of apersonal computer and the like. Advantages of the liquid crystaldisplay, such as the fact that they are thinner and lighter than theconventional display devices, for example, largely contributed to theirgain in popularity. Moreover, a narrow viewing angle in conventionalLCDs, which was previously considered to be a disadvantage, has beenrecently overcome by newly developed LCDs with a wide viewing angle.Accordingly, a wide variety of usages of the LCDs beyondpersonal-computer-use is expected to emerge.

Since a liquid crystal panel itself is not a light-emission device, theLCD needs an illumination light source. Reflection type LCDs useexternal illumination as the illumination light source. However,transmission type LCDs equipped with an internal backlight system aremore popular.

FIG. 13 shows a typical structure of a conventional transmission typeLCD. In this example, a backlight system 20 includes a light source 21,light guide 22, light diffusion elements 23, reflective sheet 24,diffusion sheet 25, and a prism sheet 26. Light emitted from lightsource 21 is incident on light guide 22. A cone-like shaped objectindicates a viewing position of the LCD. The incident light propagatesin the light guide 22 by experiencing multiple total internalreflections. A portion of the light in the light guide 22 is diffused(or scattered) upwards by light diffusion elements 23 and emerges fromthe light guide 22. A portion of the light emitted downwards from thelight guide 22 is reflected by reflection sheet 24 and is returned tothe light guide 22. The light emitted upwards from the light guide 22 isdiffused by diffusion sheet 25 and converged by prism sheet 26. Theresultant light is used as illumination light for a liquid crystal cell30, which is sandwiched by polarizing plates 31 and 32.

In most LCDs, polarized light that is obtained by transmitting lightthrough a polarizing plate is modulated in the liquid crystal layer.Approximately half of the incident light is absorbed at the polarizingplate and, accordingly, the efficiency in light usage is low. In orderto produce sufficient luminance, more light needs to be incident on thepolarizing plate. However, increasing the light intensity causes avariety of problems, such as increased power consumption of the lightsource and adverse effects on the liquid crystal material due to heatgenerated from the thus powered up light source, which degrades thedisplay quality.

To solve the above-mentioned problems, the following technique has beenproposed. First, unpolarized light from a light source is split by apolarization light splitter into two linearly polarized light beamshaving the polarization directions orthogonal to each other. Then, oneof the linearly polarized light beams is directly used for illuminationwhile the other polarized light beam is used indirectly. In other words,one of the polarization components, which are separated by thepolarizing plate, is directly incident on the liquid crystal cell,whereas the other polarization component, which progresses towards thelight source, is re-directed toward the polarizing plate by reflectionor the like. This way, the efficiency in light usage can be improved.Some of the recent developments along this direction are as follows.

(1) In Japanese Application Laid-Open Publication No. 04-184429, anunpolarized light beam from a light source is split by a polarizationlight splitter into two orthogonally polarized light beams. One of thepolarized light beams is directed towards the liquid crystal cell. Theother beam, which progresses towards the light source, is converged andthen reflected towards the liquid crystal cell.

(2) A backlight system disclosed in Japanese Application Laid-OpenPublication No. 06-265892 includes a beam deflector that transmits lightin a direction substantially normal to the light emitting surface of aplanar light guide. The deflector is disposed on the light emitting sideof the planar light guide, and a polarization light splitter is disposedabove the deflector.

(3) In a backlight system disclosed in Japanese Application Laid-OpenPublication No. 07-261122, a polarized light splitter is located on thelight emitting side of a parallel light generating device. The parallellight generating device is constructed of a light scattering conductorincluding a portion having a wedge-shaped profile.

(4) Similar systems are proposed in Japanese Application Laid-OpenPublications No. 06-289226, No. 07-49496. All the proposed polarizedlight splitting systems, including the above-mentioned (1) to (3),employ a multi-layer film utilizing the Brewster law (Brewster's angle)as polarization splitting means.

FIGS. 11 and 12 show cross-sectional views of conventional polarizationlight splitting films. In FIG. 11, polarization light splitting film 40has a multi-layer structure formed by alternately laminating lighttransmission layers 41 having a large refractive index and lighttransmission layers 42 having a small refractive index. Using Brewster'slaw, the polarization light splitting film 40 is designed to transmitthe p-polarized light component and reflect the s-polarized lightcomponent.

Polarization light splitting film 40 in FIG. 12 also has a laminatedstructure of two types of layers 43, 44 having different refractiveindices. The thickness of the layers 43 is designed to causeinterference with respect to visible light. In this construction, if therefractive indices of the layers 43, 44 and the thickness of layer 43satisfy a certain predetermined relationship, the transmission contrastbetween the p-polarized component and the s-polarized component becomeslarge with respect to a certain incident angle. The polarization lightsplitting film 40 of FIG. 12 utilizes this property to transmitpolarized light.

However, the above-mentioned conventional techniques have the followingdisadvantages. The backlight system of (1) is intended for use withprojection LCD devices. The structure of such an illumination systemrequires a large amount of space. Therefore, it cannot be applied tothin panel-type LCD devices.

The backlight system of (2) is applicable to thin LCD devices. If apolarized light splitting layer is fabricated on the inclined sides of acolumnar prism array having a plurality of triangular-shape prism units,a relatively high efficiency can be achieved in light usage. However,the structure of the polarized light splitting layer becomescomplicated. In particular, it is difficult to fabricate the polarizedlight splitter layer on the inclined sides of the columnar prism array.This type of backlight system is therefore not suitable for massproduction.

As for the backlight system of (3), if the parallel light generatingdevice is constructed of a certain light scattering conductor having awedge-shaped profile, superior efficiency in light usage can beobtained. However, it is difficult to manufacture such a lightscattering conductor with a desirable light scattering performance.Accordingly, this system is not suitable for actual use.

In addition, since all the systems discussed in (1) to (4) utilize theBrewster law (or Brewster's angle), it is necessary to form multiplelayers on an inclined face. Therefore, these systems require acomplicated structure and are not suitable for mass production.

Furthermore, to produce a sufficient polarization light splittingproperty using the conventional multi-layer structures above, at leastfive layers need to be laminated, resulting in a complicated layerstructure. In addition, the polarization light splitting propertydepends on the incident angle and the wavelength, etc., imposing manyundesirable limitations in the actual use. For example, becauseillumination light that propagates in a direction normal to the emittingsurface of the backlight system is polarized and split using Brewster'slaw, the multiple layers need to be inclined with respect to theemitting surface by forming the layers on an inclined plane. Therefore,the polarization light splitting device must be thick enough toaccommodate this structure. This is not consistent with a recent trendtowards reducing the thickness of LCD devices.

In addition, the conventional polarization light splitting devices wouldnot be effective unless the coupling of the light splitting devices withother elements of the backlight system, such as a diffusion film and aprism sheet, is optimized. Also, it is important to add anti-reflectionlayers, hard coating layers, adhesive compound/cement layers, and/orpolarizing layers to the polarized light splitting device for the wholedevice to be effective under various operating conditions. However,there have been no such proposals.

As described above, some of the conventional backlight systems use alaminated structure of a plurality of layers having different refractiveindices. Other systems use the multi-layered structure formed on aprism-like shaped object. In any event, the conventional backlightsystems are limited in the sense that they use the Brewster law(Brewster's law).

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a polarization lightsplitting film that substantially obviates the problems due tolimitations and disadvantages of the related art.

An object of the present invention is to provide a thin, highlyefficient polarization light splitting film having a relatively simplestructure and being suitable for mass production.

Another object of the present invention is to provide a polarizationlight splitting film for use in a backlight system that does not requirea complicated coupling with other elements of the backlight system.

A further object of the present invention is to provide a thin-typebacklight and an LCD device that utilizes a polarization light splittingfilm of the present invention.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, thepolarization light splitting film according to an aspect of the presentinvention includes an optical rotation selection layer that reflects oneof the right and left circular polarization components of incident lightand transmits the other component of the light. The polarization lightsplitting film also includes a quarter-wave plate. These two layerstogether perform a similar function to that of the conventionalpolarization light splitting film that transmits linearly polarizedlight. The reflected light of the polarization light splitting film ofthe present invention is a circularly polarized beam. When the opticalrotation face of the polarization light splitting film is used for alight receiving surface and the quarter-wave plate side face is used fora light transmitting surface, either the right or left circularpolarization component of the incident light is reflected and the othercomponent is transmitted at the optical rotation selection layer. Thetransmitted light is converted into linearly polarized light at thequarter-wave plate, and is emitted from the polarization light splittingfilm as linearly polarized light. A cholesteric liquid crystal layer maybe used for the optical rotation selection layer. According to thepresent invention, the efficiency in light usage of the backlight systemutilizing the polarization light splitting film of the present inventioncan be improved.

A diffusion layer, prism layer, and/or lens layer may additionally beformed on the light receiving side of the polarization light splittingfilm of the present invention. In this case, the polarization lightsplitting film possesses the corresponding functions of diffusion,deflection, and/or convergence.

Also, by adding an anti-reflection layer on the incident surface,undesirable reflection at the incident surface can be prevented.Furthermore, a polarizer layer may be laminated on the emitting surfaceof the polarization light splitting film to generate linearly polarizedlight with a higher linearity.

A backlight system according to another aspect of the present inventionincludes the above-described polarization light splitting film, aplate-like light guide placed on the incident surface side of thepolarization light splitting film, and a light source placed on the sideof the light guide.

An LCD device according to a further aspect of the present inventionincludes the above-described backlight as a back light source of aliquid crystal cell.

In another aspect, the present invention provides a polarization lightsplitting film having a light receiving side and a light transmittingside, the polarization light splitting film including an opticalrotation selection layer at the light receiving side for reflecting oneof right and left circularly polarized components of a light beam thatis incident thereon from the light receiving side and for transmittingthe other one of the right and left circular polarization components ofthe light beam; and a quarter-wave layer laminated over the opticalrotation selection layer at the light transmitting side.

In another aspect, the present invention provides a backlight systemincluding a polarization light splitting film having a light receivingside and a light transmitting side, the polarization light splittingfilm including an optical rotation selection layer at the lightreceiving side for reflecting one of right and left circularly polarizedcomponents of a light beam that is incident thereon from the lightreceiving side and for transmitting the other one of the right and leftcircular polarization components of the light beam, and a quarter-wavelayer laminated over the optical rotation selection layer at the lighttransmitting side; a light source for emitting a light beam; and a lightguide disposed at the light receiving side of the polarization lightsplitting film for guiding the light beam emitted from the light sourcetowards the polarization light splitting film as the incident light.

In a further aspect the present invention provides a liquid crystaldisplay device including a backlight system and liquid crystal cellreceiving the light transmitted from the backlight system to displayimages. The backlight system includes a polarization light splittingfilm having a light receiving side and a light transmitting side, thepolarization light splitting film including an optical rotationselection layer at the light receiving side for reflecting one of rightand left circularly polarized components of a light beam that isincident thereon from the light receiving side and for transmitting theother one of the right and left circular polarization components of thelight beam, and the polarization light splitting film further includinga quarter-wave layer laminated over the optical rotation selection layerat the light transmitting side; a light source for emitting a lightbeam; and a light guide disposed at the light receiving side of thepolarization light splitting film for guiding the light beam emittedfrom the light source towards the polarization light splitting film asthe incident light.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is a cross-sectional view of a polarization light splitting filmaccording to a preferred embodiment of the present invention;

FIG. 2 is a cross-sectional view of a polarization light splitting filmaccording to another preferred embodiment of the present invention;

FIG. 3 is a cross-sectional view of a polarization light splitting filmhaving a diffusion layer according to another preferred embodiment ofthe present invention;

FIG. 4 is a cross-sectional view of a polarization light splitting filmhaving a prism layer according to another preferred embodiment of thepresent invention;

FIGS. 5A and 5B are perspective views of various examples of the prismlayer for use in the polarization light splitting film of FIG. 4;

FIG. 6 is a cross-sectional view of a polarization light splitting filmhaving a lens layer according to another preferred embodiment of thepresent invention;

FIGS. 7A, 7B, and 7C are perspective views of various examples of theprism layer for use in the polarization light splitting film of FIG. 6;

FIG. 8A is a cross-sectional view of a polarization light splitting filmhaving an anti-reflection layer according to another preferredembodiment of the present invention;

FIG. 8B is a cross-sectional view of a polarization light splitting filmhaving a polarizing layer according to another preferred embodiment ofthe present invention;

FIG. 9 is a cross-sectional view of a liquid crystal display devicehaving a backlight system according to another preferred embodiment ofthe present invention;

FIG. 10 is a cross-sectional view of a liquid crystal display devicehaving a backlight system according to still another preferredembodiment of the present invention;

FIG. 11 is a cross-sectional view of a conventional light splittingfilm;

FIG. 12 is a cross-sectional view of another conventional lightsplitting film;

FIG. 13 is a cross-sectional view of a conventional liquid crystaldisplay device having a backlight system; and

FIG. 14 is a schematic diagram showing the experimental set-up formeasuring the degree of linear polarization for each of the workingexamples of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

FIGS. 1-4, 6 and 8 show cross-sectional views of various preferredembodiments of the polarization light splitting film according to thepresent invention. In FIG. 1, a polarization light splitting film 10 ofthe present invention includes a quarter-wave layer 2 (λ/4 phasedifferentiation layer) formed on an optical rotation selection layer 1.When an unpolarized light beam is incident on the optical rotationselection layer 1 from below, one of the right and left circularlypolarized components of the incident light beam is reflected and theother component is transmitted through the optical rotation selectionlayer 1. The circularly polarized light transmitted through opticalrotation selection layer 1 becomes a linearly polarized light beamthrough the quarter-wave layer 2 and is emitted from the polarizationlight splitting film 10.

As the optical rotation selection layer 1, any kind of material and/orstructure may be used as long as the material and the structure has thedesired selectivity on circular polarization. For example, a liquidcrystal, in particular, cholesteric liquid crystal, may be used as thematerial to form the optical rotation selection layer 1.

Cholesteric liquid crystal exhibits the above-mentioned optical rotationselectivity due to its helical molecular structure. Light incident at anangle parallel to the helical axis of the planer structure of thecholesteric liquid crystal is split into two circularly polarized lightwaves with different rotation directions (right and left: clockwise andcounter-clockwise). One of the two circularly polarized components istransmitted and the other component is scattered and reflected. Thisphenomena is known as circular dichroism. When the orientation of theoptical rotation of the circularly polarized light matches the incidentlight, the circularly polarized light that has the same orientation asthe optical rotation of the helical axis of the cholesteric liquidcrystal is selectively reflected. The maximum reflection of theoptically rotated beam occurs at the wavelength λ₀ in accordance withthe following Equation (1).

λ₀ =n _(AV) ·p  (1)

Here, p is the helical pitch, and n_(AV) is an average refractive indexin a plane perpendicular to the helical axis. The wavelength bandwidthΔλ of the reflected light is given in the following Equation (2).

Δλ=Δn·p  (2)

where Δn=n∥−n⊥, n∥ is the maximum refractive index in a planeperpendicular to the helical axis, and n⊥ is the maximum refractiveindex in a plane parallel to the helical axis. Also, it is known thatthe wavelength λφ of the selectively scattered light, originating fromthe light obliquely incident on the liquid crystal layer with respect tothe helical axis of the planer structure, is shifted from λ₀ toward theshorter wavelength.

Using the above-mentioned characteristics, it is possible to use onlylight having a certain wavelength. Alternatively, the helical pitch pand the refractive index difference Δn of the cholesteric liquid crystalmay be adjusted such that the wavelength bandwidth Δλ of the reflectedlight covers the entire range of visible light, making it possible touse white light.

As such a cholesteric liquid crystal, it is desirable to use chiralnematic liquid crystal compounds in which an optically active substance,such as a 2-methylbutyl group, 2-methylbutoxy group, or 4-methylhexylgroup, is attached to the end group of a nematic liquid crystalcompound, such as a schiff base, azo-system, ester-system, orbiphenyl-system.

Also, cholesteric liquid crystal polymers are among the preferablematerials for the cholesteric liquid crystal of the present invention.This is because the cholesteric liquid crystal polymer is in its solidphase at a room temperature, and its chiral characteristics are easilymaintained. In general, to form a liquid crystal polymer, a mesogengroup of a liquid crystal phase is introduced in the main chain and/orthe side chain. The cholesteric liquid crystal polymer can also beobtained by introducing a cholesteril group in the side chain, forexample. In this case, the cholesteril group can be introduced to theside chain of a main chain polymer of polysiloxane or a vinyl polymer,such as an ethyiene-vinyl acetate copolymer, through a spacer(molecular) providing an appropriate distance. Alternatively, a vinylmonomer having a cholesteril group may be polymerized to produce thecholesteric liquid crystal polymer.

The optical rotation selection layer of the present invention may have asingle layered structure of a cholesteric liquid crystal layer or amulti-layer structure in which an optical rotation selection layer iscoated on a base member (substrate). In the polarization light splittingfilm 10 shown in FIG. 2, optical rotation selection material, made ofcholesteric liquid crystal, is coated on a base member 3 (substrate) toform a cholesteric liquid crystal layer 4 thereon. The optical rotationselection layer 1 corresponds to the multi-layered structure of basemember 3 and cholesteric liquid crystal layer 4. In addition, aquarter-wave layer 2 is formed on the cholesteric liquid crystal layer4. The substrate side is used as the light receiving surface, and thequarter-wave layer side is used as the light transmitting surface. In analternative construction, the substrate 3 may be disposed between thequarter-wave layer 2 and the cholesteric liquid crystal layer 4. Thearrangement of FIG. 2 has a certain advantage in that the substrate 3can play the role of a light diffusion layer, prism layer, and/or lenslayer, as will be described below. On the other hand, if the substrateis placed between the quarter-wave layer 2 and the cholesteric liquidcrystal layer 4, the substrate 3 preferably has little or no opticalanisotropy to ensure that light originating from the optical rotationselection layer 4 is conducted towards quarter-wave layer 3 without anyundesirable optical modifications.

These multi-layer structures are useful when the optical rotationselection layer is too thin to be handled with ease or when there isdifficulty in laminating the optical rotation selection layer togetherwith the quarter-wave layer. In addition, with the multi-layer of thecholesteric liquid crystal layer and the substrate of FIG. 2, thesurface of the substrate 3 can be adapted to perform molecularorientation control, i.e., alignment of the orientation of thecholesteric liquid crystal molecules. Typically, the helical axis isaligned normal to the emitting surface of the polarization lightsplitting film.

In the case of the single layered structure of FIG. 1, the molecularorientation control can be performed by forming the cholesteric liquidcrystal layer on a separate substrate that will not become a constituentelement of the polarization light splitting film. The molecularorientation is controlled on the separate substrate, and then theresultant cholesteric liquid crystal layer is peeled off from thesubstrate. Typically, the above-mentioned orientation control isperformed by applying cholesteric liquid crystal material on the rubbedsurface of a resin film formed on a substrate (either as part of thelight splitting film or separate).

A resin film can be used as the substrate 3. The desirable properties ofsuch a resin film include superior transparency, dimensional stability,and coating ability. In particular, films made frompolyethyleneterephtalate, polyethylenenaphtalate, polycarbonate,polyethersulfone, and/or acrylic resin are preferable.

The quarter-wave layer 2 may be formed using various methods: laminatingmultiple layers having different refractive indices, providing aspecific orientation in liquid crystal, or orienting molecules bystretching in a predetermined direction a resin film made of, forexample, polyesters such as polyethyleneterephtalate, polyethersulfone,polycarbonate, polysulfone, polyetheretherketone, or polyetherketone.Typically, the thickness of the quarter-wave layer 2 is set toapproximately 8 to 100 μm.

As shown in FIGS. 3, 4, 6, and 8, the polarization light splitting filmof the present invention may include additional functional layerslaminated on the light receiving surface or light emitting surface.

Polarization light splitting film 10 shown in FIG. 3 includes adiffusion layer 5 laminated on the light receiving side. When thepolarization light splitting film 10 is used in a backlight system foran LCD device, the diffusion layer 5 serves to blur the image of thelight source and thus improve the visibility of the display device. Inaddition, with the diffusion layer 5, reflected light can be re-used inthe polarization light splitting film itself. In other words, asdescribed above, when unpolarized light is incident on the opticalrotation selection layer, one of the right and left circularly polarizedcomponents is reflected and the other component is transmitted throughthe optical rotation selection layer. The reflected circularly polarizedwave is scattered in the diffusion layer and changed into unpolarizedlight, and the unpolarized light returns to the optical rotationselection layer. By repeating these steps, a highly polarized beam canefficiently be obtained.

The diffusion layer 5 may be formed by dispersing diffusive agents in alight transmitting resin, utilizing the optical scattering effect due tothe difference in refractive index between the diffusion agent and thelight transmitting resin, or by forming an uneven surface on a lighttransmitting resin. Examples of the diffusion agent are beads, fillersand hollow beads with the main component being an acrylic resin,polystyrene, polyethylene, ureaformaldehyde resin, urethane resin,organic silicon resin, calcium carbonate, titanium oxide, or silica. Itis desirable that the average particle diameter of the diffusion agentbe 1 to 50 μm to facilitate handling. Also, two or more types of agent,with different particle diameters, may be used. Examples of the lighttransmitting resin are a polyester system resin, acrylic system resin,polystyrene system resin, polyvinylchloride system resin,polyvinylidenechloride system resin, polyethylene system resin,polypropylene system resin, polyurethane system resin, polyamide systemresin, polyvinylacetate system resin, polyvinylalcohol system resin,epoxy system resin, cellulose system resin, silicon system resin,polyimide system resin, polysulfone system resin, and polyarylate systemresin.

Polarization light splitting film 10 in FIG. 4 has a prism layer 6laminated on the light receiving side. The prism layer 6 is made of aplurality of prism units. In order for the prism layer 6 to deflect theincident light and change the traveling direction, each of the prismunits desirably is a triangular column with a triangular cross-sectionlike prism layer 6 shown in FIG. 5A or a pyramid-shape prism layer 6 asshown in FIG. 5B. The pyramid is a quadrangular pyramid.

Polarization light splitting film 10 shown in FIG. 6 has a lens layer 7laminated on the light receiving side. The lens layer 7 is made of aplurality of lens units. In order for the lens layer to deflect theincident light and converge or diffuse the light, each of the lens unitsdesirably is a semi-circular column shape lens layer 7 as shown in FIG.7A, an inverse semicircular column shape lens layer 7 as shown in FIG.7B, or a dome-shape lens layer 7 as shown in FIG. 7C.

The above-mentioned prism layer and lens layer can be formed by variousmethods: a thermal press method in which the uneven shape is molded onthe specimen's surface by pressing a high-temperature molding form on aresin film in a thermoplastic state; a simultaneous extrusion-embossingmethod in which, when a resin is formed into a film shape bymelting-extrusion at a high temperature, an uneven shape is concurrentlyformed on the surface by an embossing roll; or a 2P (Photo Polymer)method in which an ionizing radiation-setting resin is molded on a resinfilm by a mold form and then cured. The resultant film with the unevensurface is adhered to the optical rotation selection layer with anadhesive or is used as the substrate.

The resin film that can be used in the above-mentioned thermal pressmethod and simultaneous extrusion-embossing method includes, forexample, acrylic resins, polycarbonate resins, polyester resins such aspolyethyleneterephtalate, polystyrene resin, and vinylchloride resin.

Examples of the ionizing radiation-setting resin that can be used in theabove-mentioned 2P method include, for example, knownultraviolet-setting resins and electron beam setting resins, representedby compounds made from a prepolymer and/or a monomer that has in itsmolecules a functional polymerization group, such as a (meta)-acryloylgroup (here and in the following description, “(meta)” is used torepresent both with and without “meta”, i.e., acryloyl group andmeta-acryloyl group), a (meta)acryloyloxy group, epoxy group, or amercapto group.

For example, the main component of such a compound is a prepolymer, suchas urethane(meta)acrylate, epoxy(meta)acrylate, orsilicon(meta)acrylate, or a multifunctional monomer, such astrimethylolpropanetri(meta)acrylate, ordipentaerythritolhexa(meta)acrylate.

In order for the prism layer or lens layer to have a high refractiveindex, it is preferable to use a high bridged type compound having theabove-mentioned multifuctional monomer as the principle component.Typically, a non-solvent type ionizing radiation setting resin is used.However, depending on requirements, it is acceptable to use the one thatis diluted in a solvent.

Examples of the above-mentioned resin film that can be used togetherwith the ionizing radiation setting resin include a polyester resin suchas polyethyleneterephtalate, an acrylic resin such aspolymethyl(meta)acrylate, and films made of polycarbonate, polyarylate,polyimide, polypropylene, or fluororesin.

To cure the ionizing radiation setting resin with ultraviolet radiation,a photopolymerization initiator, such as an acetophenone class orbenzophenone class, and/or a photosensitization agent, such asn-butylamine, triethylamine or tri-n-butylphosphine, are appropriatelyadded in the resin.

Curing of the ionizing radiation setting resin is normally conducted byirradiating the resin with an electron beam or ultraviolet radiation.When the electron beam is used for the curing, the electron beam needsto have an energy level ranging from 50 to 1000 keV, or more preferablyfrom 100 to 300 keV. Such an electron beam can be obtained from anelectron beam accelerator, such as Cockroft-Walton's type, Van De Graaftype, resonance transformation type, insulated core transformation type,linear type, dynamitron type, or high-frequency type. Forultraviolet-setting, ultraviolet rays from a light source, such as avery high-pressure mercury-arc lamp, a high-pressure mercury-arc lamp, alow-pressure mercury-arc lamp, a carbon arc, a xenon arc, or a metalhalide lamp, can be used.

Referring to FIG. 8A, it is possible to laminate an anti-reflectionlayer 11 on the optical rotation selection layer, diffusion layer, prismlayer, or the lens layer at the respective light receiving side. Withthe anti-reflection layer, the loss of light due to reflection on theincident surface can be reduced.

A standard anti-reflection layer may be used for this purpose.Alternatively, a multi-layer film formed by alternately laminating lowrefractive index layers and high refractive index layers with the lowrefractive index layer on the front side may be used as theanti-reflection layer. Such low refractive index layers may be formed byvacuum deposition, sputtering, or plasma chemical vapor deposition (CVD)of a low refractive index material, or by coating a coating solutionhaving a powder-state low refractive index material in a resin binder.The high refractive index layers may be formed by vacuum deposition,sputtering, or plasma CVD of a high refractive index material, or bycoating a coating solution having a powder-state high refractive indexmaterial in a resin binder. In view of the anti-reflection effect, it isdesirable that the thickness of the low refractive index layer be about0.1 μm and that the thickness of the high refractive index layer also beabout 0.1 μm.

If the surface on which the anti-reflection layer is formed is an unevensurface, such as a prism surface or lens surface, the anti-reflectionlayer may be formed by a coating method, such as a dipping method orspraying method, or by a thin-film forming method, such as vacuum vapordeposition, sputtering, or plasma CVD.

For the low refractive index material, inorganic compounds, such as LiF,MgF₂, 3NaF.AlF₃, AlF₃, Na₃.AlF₆ or SiOx (1.50≦x≦2.00) (refractive indexof 1.35 to 1.48), may be used. When included in the resin binder,ultra-micro particles with a diameter of 5 nm to 50 nm may be used.

In addition, a low refractive index organic compound, such as a lowrefractive index thermoplastic resin (polymer), may be added to theresin binder in addition to the above-mentioned ultra-micro particles toadjust the refractive index of the low refractive index layer. Examplesof such a low refractive index thermoplastic resin are fluororesinsobtained by homopolymerization of fluorine inclusive monomers, such asCF₂═CF₂, CH₂═CH₂, and CF₂═CHF, or by copolymerization, blockpolymerization or graft polymerization of two or more kinds of monomers.For example, PTFE (poly4ethylene fluoride), PVDF (polyvinylidenefluoride), and PVF (polyvinyl fluoride) are desirable because theirrefractive index is low: 1.45 or less. As the low refractive indexmaterial described above, it is also possible to make the resin binderitself have a low refractive index by including fluoride in themolecules of the compound that constitutes the resin binder.

For the high refractive index material, inorganic compounds, such asZnO, TiO₂, CeO₂, Sb₂O₅, SnO₂, ITO (Indium Tin Oxide), Y₂O₃, La₂O₃, ZrO₂,or Al₂O₃, can be used. When the material is to be included in a resinbinder, the corresponding ultra-micro particles are used.

In addition, when a resin binder is used for the high refractive indexlayer, the resin binder itself can have a high refractive index byincluding the halogen group (except fluoride), sulfur, nitrogen, orphosphorus in the molecules of the compound that constitutes the resinbinder.

An ionizing radiation setting resin can be used as a resin binder forthe formation of the low refractive index layer and the high refractiveindex layer. Known ionizing radiation setting resins includesultraviolet setting resins or electron beam setting resins, representedby, for example, compounds made from prepolymers and/or monomers thathave a polymerization function group, such as a (meta)acryloyl group,(meta)acryloyloxy group, epoxy group, or mercapto group, in theconstituent molecules. Examples of compounds that can be used are madeby adding an appropriate reactive dilution to a prepolymer, such asurethane(meta)acrylate, epoxy(meta)acrylate, or silicon(meta)acrylate.The reactive dilution may be a monofunctional monomer, such asethyl(meta)acrylate, ethylhexyl(meta)acrylate, styrene, methystyrene, orN-vinylpyrrolidene, or a multifunctional monomer, such astrimethylolpropanetri(meta)acrylate, hexanediol(meta)acrylate,tripropyleneglycoldi(meta)acrylate, diethyleneglycoldi(meta)acrylate,pentaerythritoltri(meta)acrylate, dipentaerythritolhexa(meta)acrylate,1,6-hexanedioldi(meta)acrylate, and neopentylglycoldi(meta)acrylate.

For producing a low refractive index, it is desirable to have lessmultifunctional monomers, which are desirable to use if the film lacks asufficient elasticity. When a fluoride inclusive monomer, such astrifluoroethylacrylate (refractive index of 1.32), is used, the resinbinder itself can have a low reflective index. Trifluoroethylacrylate isa monofunctional monomer, and when its homopolymer has a weak filmstrength, a multifunctional monomer, such asdipentaerythritolhexaacrylate, may be added. However, when themultifunctional monomer is added, the refractive index increases.Therefore, the concentration of such a multifunctional monomer isdesirably within the range of 1 to 50 parts by weight for every 100parts by weight of the monofunctional monomer, or more preferably,within the range of 5 to 20 parts by weight.

The above-mentioned ionizing radiation setting resin may be used withouta solvent or diluted in a solvent, depending on the need.

An example of the ionizing radiation setting resin is a mixture ofpolyester acrylate and polyurethane acrylate. The resultant film has astrength, flexibility, and impact durability. The mixing ratiopreferably is 30 or less parts by weight of polyurethaneacrylate forevery 100 parts by weight of polyesteracrylate to produce sufficienthardness.

In addition, when a setting resin is cured with ultraviolet rays, in afashion similar to the ionizing radiation setting resin used in the lenslayer and prism layer described above, a photopolymerization initiatorand photosensitization agent can be appropriately added to the resin.The curing process can be conducted by irradiating the resin withelectron beams or ultraviolet rays in a similar manner.

FIG. 8B shows a polarization light splitting film 10 according toanother preferred embodiment of the present invention, in whichpolarizing layer 8 is laminated on the emitting side of the polarizationlight splitting film. In FIG. 8B, polarizing layer 8 is laminated onquarter-wave layer 2. The light emitted from the quarter-wave layer isconverted from a circularly polarized beam to a linearly polarized beamby the quarter-wave layer 2. By incorporating polarizing layer 8, evengreater linearity in the polarization can be obtained; i.e., a linearlypolarized beam having strong linear polarization. A polarizing film maybe used as the polarization layer 8.

Known resin based films can be used as such a polarizing film. Forexample, it is possible to form the film using a method in which iodineand/or a dichroistic dye is adsorbed and oriented on a hydrophilic film,such as a polyvinylalcohol system film or a partially formalpolyvinylalcohol film, an ethylene-vinylacetate copolymer systemsaponificated film, and a cellulose system film, or using a method offorming polyene by performing a dehydration process on apolyvinylalcohol system film.

By adding the polarizing layer 8 to the polarization light splittingfilm, as described above, a polarizer plate on the light incident sideof a liquid crystal cell, which is normally required to produce alinearly polarized light beam, can be omitted. In this case, it is ofcourse necessary to match the polarization axis of polarizing layer 8with the polarization axis of the linearly polarized light emitted fromthe quarter-wave layer 2.

If a polarization light splitting film does not include a polarizinglayer, as in the previous embodiments, the polarization axis of thequarter-wave layer (the polarization axis of linearly polarized lightemitted from that layer) should be matched with the polarization axis ofa liquid crystal cell that is to be coupled to the polarization lightsplitting film.

The above-mentioned layers may be laminated on a base member having asufficient strength such as substrate 3 in FIG. 2. Also, it is possibleto adhere the layers together using a cement layer or an adhesive layerin between.

For the cement used for the cement layer, it is desirable to useurethane system cements, for example, reactive-setting type urethanesystem cements, such as humidity-setting types (1-liquid type) orthermosetting types (2-liquid type). The humidity-setting types are usedin combination with an oligomer and/or a prepolymer of a polyisocyanatecompound. The thermosetting-types are used in combination with anoligomer and/or a prepolymer of a polyol compound and with a monomer,oligomer, and/or prepolymer of a polyisocynate compound. For thesereactive-setting-type urethane system cements, when the aging process isconducted after the lamination, it is desirable to conduct it in thetemperature range from a room temperature to 80 degrees centigrade. Forthe adhesive used for the adhesive layer, known adhesive compounds ofacrylic resin-type can be used.

On the polarization light splitting film with the laminated structure, ahard coat layer, an adhesive layer, or a cement layer can be formeddepending on their needs. Here, the hard coat layer represents a layerhaving a hardness of H or higher in the pencil hardness test based onJIS (Japanese Industrial Standards) K5400. The hard coat layer can beformed from either inorganic materials or organic materials. An exampleof an inorganic material that can be used is a complex oxide film formedby a sol-gel method. Organic materials that can be used include atransparent resin film of a thermoplastic resin, and more preferably, areactive-setting resin, such as a thermosetting resin or ionizingradiation setting resin. The ionizing radiation setting resin is morepreferable. For the thermosetting resin, a phenol resin, urea resin,diallylphtalate resin, melamine resin, guanamine resin, unsaturatedpolyester resin, polyurethane resin, epoxy resin, aminoalkyd resin,melamine-urea copolycondensated resin, silicon resin, or polysiloxaneresin can be used. Depending on the requirements of these resins, thehard coat layer is coated with a coating solution that includes abridging agent or polymerization initiator as a setting agent, apolymerization accelerator, a solvent, and a viscosity adjustment agent.

For the ionizing radiation setting resin, the resins used for the prismlayer, lens layer, low refractive index layer, and high refractive indexlayer described above can be used. To add flexibility to the hard coatlayer of the ionizing radiation setting resin, it is acceptable toinclude 1 to 100 parts by weight of the thermoplastic resin for every100 parts by weight of the ionizing radiation setting resin. For thethermoplastic resin, if the ionizing radiation setting resin compositionis made of polyester acrylate and polyurethane acrylate,polymethylmethacryl acrylate and polybutylmethacryl acrylate aredesirable in terms of their transparency, low haze value, high lighttransmitability, and compatibility. When the resin is cured withultraviolet radiation, a photopolymerization initiator and aphotosensitization agent are appropriately added in the resin in asimilar manner to the ionizing radiation setting resin used in the lenslayer and prism layer. Similarly, the curing is conducted by irradiationwith electron beams or ultraviolet rays. In addition, it is preferablethat the thickness of the hard coat layer be about 0.5 μm, or morepreferably, about 3 μm or more to maintain necessary hardness.

As the adhesive layer and the cement layer, known materials, such asadhesives with an acrylic resin and cement layers with a urethane systemadhesive, can be used, for example.

Next, backlight and liquid crystal devices according to other preferredembodiments of the present invention will be described with reference toFIGS. 9 and 10. FIG. 9 shows a cross-sectional view of the backlight andthe liquid crystal device using the polarization light splitting filmaccording to another preferred embodiment of the present invention. FIG.10 shows a cross-sectional view of a backlight and liquid crystal deviceaccording to still another preferred embodiment of the presentinvention.

In FIG. 9, backlight system 20 includes light source 21, light guide 22,light diffusion elements 23, reflection sheet 24, and polarization lightsplitting film 10. The liquid crystal device includes the backlightsystem 20, liquid crystal cell 30, and polarizing plate 31 affixed tothe front side of the liquid crystal cell 30.

In the backlight and liquid crystal device of FIG. 10, the polarizationlight splitting film 10 is affixed to liquid crystal cell 30 by anadhesive or cement. In FIG. 10, backlight system 20 includes lightsource 21, light guide 22, light diffusion elements 23, reflection sheet24, and polarization light splitting film 10 in contact with liquidcrystal cell 30. The liquid crystal device includes the backlight system20, liquid crystal cell 30, and polarizing plate 31 affixed to the frontsurface of the liquid crystal cell 30.

In FIGS. 9 and 10, an additional polarizing plate may be formed on thebottom surface (light incident surface) of the liquid crystal cell 30.Also, if desired, a diffusion sheet or prism sheet may be insertedbetween light guide 22 and polarization light splitting film 10. Here,as the light source 21, a linear light source, for example, a coldcathode tube, may be used.

The material of light guide 22 needs to have the ability to transmitlight efficiently but is not limited to a specific class of material.Examples are an acrylic resin, such as PMMA (polymethylmeta acrylate), apolycarbonate resin, and glass. The light guide 22 preferably has aplate-shape or a wedge-shape with the thickness decreasing in adirection away from the light source. Light emitted from the lightsource 21 travels in the light guide 22 by repeating total internalreflections. The light is also diffused (scattered) by the lightdiffusion elements 23, and is emitted from the emitting face of thelight guide 22.

To form the light diffusion elements 23 in the light guide, thefollowing methods can be used: (1) forming light diffusion elements bydispersing such an agent as silica in a resin in a dot pattern on thelight emitting face or on the other (bottom) face of the light guide 23by a printing or like method (FIGS. 9 and 10 shows the case of havingthe diffusion elements on the bottom face of light guide 22); (2)roughening the emitting face of the light guide 23; (3) roughening thebottom face of light guide 23; or (4) introducing a light diffusion(scattering) agent, such as a resin, having a different refractive indexfrom the light guide into the light conducting material of the lightguide.

For reflective sheet 24, a white plastic sheet or a plastic sheet havingaluminum vapor-deposited thereon, can be used.

In the structure shown in FIGS. 9 and 10, a diffusion film, prism sheetand/or polarizing plate can be placed between the light guide and thepolarization light splitting film in a similar manner to conventionalbacklight systems which do not have a polarization light splitting film.Such a diffusion film is desirable when a polarization light splittingfilm does not include a diffusion layer on the incident surface side.Also, a prism sheet or polarizing plate is desirable when thepolarization light splitting film does not include the correspondinglayer (prism layer or polarizing layer).

As for the arrangement of the diffusion film and prism sheet, instead ofplacing the diffusion film and prism sheet in that order from theemitting surface side of the light guide, the order of the arrangementmay be the prism sheet and then the diffusion film. Multiple layers ofdiffusion film and prism sheets may also be used. The arrangement of thediffusion films and the prism sheets is not limited by use of thepolarization light splitting film of the present invention.

The diffusion film may be formed by molding a polymer film using theembossing method or the like for forming a rough surface or bydistributing beads or fillers of an organic or inorganic material in theresin.

The prism sheets function properly, as long as the material hasefficient light transmissibility. Examples of such a material includepolycarbonate (PC), polyester resins such as polyethylene terephtalate(PET), acrylic resins such as polymethyl metacrylate (PMMA), and glass.In particular, prism sheets created by forming a prism-shape layer on abase film made of PET or PC using an ultraviolet-setting resin arepreferable since micro prism-shape structure can be accurately formedand this forming method is suitable for mass production. In this case,the refractive index of the prism-shape layer made of theultraviolet-setting resin is preferably about 1.4 or larger, or morepreferably, about 1.5 or larger. To form the prism shape layer with theultraviolet-setting resin on the base film, it is desirable to utilize amethod disclosed in Laid-Open Japanese Patent Application No. 05-169015.In this method, the ultraviolet-setting resin is coated on an intaglioroll that has recessed patterns corresponding to the prism shape to bemanufactured, and a base film is pressed on the intaglio roll having theresin solution on the printing surface. Then, the intaglio roll isirradiated with ultraviolet radiation to cure the resin. Thereafter, thecured ultraviolet-setting resin is peeled off from the intaglio rolltogether with the base film. By this method, the prism sheet can becontinuously manufactured.

As described with reference to FIGS. 9 and 10, the liquid crystal deviceof the present invention is a display device equipped with thepolarization light splitting film in which the above-mentioned structureis used as a backlight system for illuminating the liquid crystal cell.

Working examples of the present invention will now be described withreference to the drawings.

FIRST WORKING EXAMPLE

As the optical rotation selection layer, a cholesteric liquid crystalpolymer film (having the helical axis of the liquid crystal along adirection of the thickness and showing superior circular dichroism withrespect to visible light) with a thickness of 15 μm was formed byphotopolymerization with ultraviolet irradiation. Aphase-differentiation film made by Sumitomo Chemical Co., Ltd. productname “SUMILITE” with a thickness of 60 μm, was used as the quarter-wavelayer. The resultant optical rotation selection layer and quarter-wavelayer were adhered to each other with an adhesive compound to form thepolarization light splitting film. See FIG. 1 for the structure.

SECOND WORKING EXAMPLE

A cholesteric liquid crystal polymer film with a thickness of 2 μm wasformed on a glass base by photopolymerization with ultravioletirradiation. By grinding the cholesteric liquid crystal polymer film, afiller of the cholesteric liquid crystal polymer, each molecule having afish-scale-like shape with about 20 μm in average diameter, wasobtained. The filler was dispersed into a solvent solution of acrylicresin to yield a coating solution. The coating solution was coated on apolyethyleneterephtalate film base of a thickness of 50 μm to form acholesteric liquid crystal layer of thickness 20 μm (in dry) thereon. Anoptical rotation selection layer of this working example corresponds tothe film base and the cholesteric liquid crystal layer. The quarter-wavefilm of the first working example above was adhered on the cholestericliquid crystal layer in a manner similar to that of the first workingexample to complete the polarization light splitting film of the presentworking example. See FIG. 2 for the structure.

THIRD WORKING EXAMPLE

In a third working example of the polarization light splitting film, adiffusion layer with an average thickness of 20 μm (in dry) wasadditionally formed on the polyethyleneterephtalate film side (lightreceiving side) surface of the optical rotation selection layer of thesecond working example above. The diffusion layer was formed by coatinga diffusion coat on the surface using a rolling coater. The diffusioncoat was prepared mixing acrylic resin beads with the average particlediameter of 10 μm as a diffusion agent with a polyester resin, anisocyanate setting agent, and a solvent. See FIG. 3 for the structure.

FOURTH WORKING EXAMPLE

A polarization light splitting film according to a fourth workingexample of the present invention has an anti-reflection layer attachedto the light receiving surface (polyethyleneterephtalate film surface)of the polarization light splitting film of the second working exampleabove. The anti-reflection layer was manufactured by forming a hard coatlayer with a thickness of 5 μm having a high refractive index on thepolyethyleneterephtalate film surface of the second working example andthen forming a low refractive index layer with a thickness of 0.1 μm onthe hard coat layer. To form the hard coat layer, a solution of acrylatehaving a carboxyl group with ultra-micro particles of ZrO₂ was coated onthe polyethyleneterephtalate film surface and is cured with electronbeam irradiation. The low refractive index layer was formed bydepositing SiO₂ (refractive index: about 1.46) on the hard coat layer byvapor deposition.

FIFTH TO NINTH WORKING EXAMPLES

Various film bases having uneven surface profiles were manufactured toserve as a prism layer or lens layer for supporting the optical rotationselection layer, as follows.

First, an uneven surface profile was formed on a copper plated surfaceof a cylindrical block with a plurality of unit elements arranged at a50 μm pitch and by plating the resultant surface with chrome. Theresultant cylindrical block serves as an intaglio roll for forming afilm base. Each of the unit elements on the intaglio roll has apredetermined surface profile that is in a reciprocal relationship withthe surface profile of each of the unit elements disposed on the filmbase to be manufactured. Various intaglio rolls were manufactured toform various prism layers and lens layers. Cutting processes with adiamond bit were used for forming the surface profiles of an isoscelestriangle with an apex angle of about 90 degrees (corresponding to afifth working example: see FIG. 5A), a semi-circular column shape(corresponding to a sixth working example: see FIG. 7A), and an inversesemi-circular column shape (corresponding to a seventh working example:see FIG. 7B), before plating chrome on the respective cylindricalblocks. A machine engraving processor is used for forming thequadrangular pyramid shape (corresponding to an eighth working example:see FIG. 5B) and a dome shape (corresponding to a ninth working example:see FIG. 7C) before plating chrome on the respective cylindrical blocks.

Using the thus manufactured rolling intaglios as embossing plates, therespective film bases each serving as a prism layer or a lens layer werecreated by embossing a polycarbonate film having a thickness of 100 μmby thermal pressing. Using each of the film bases as the substrate 3 forsupporting the optical rotation selection layer of the second workingexample (with the cholesteric liquid crystal layer formed on the surfaceof the film base opposite to the uneven surface), the correspondingworking examples (fifth to ninth) were completed.

TENTH TO FOURTEENTH WORKING EXAMPLES

In these working examples, film bases having uneven surface profilescorresponding to the fifth to ninth working examples above weremanufactured by a different method. The tenth to fourteen workingexamples respectively correspond to the fifth to ninth working examples.

In these working examples, instead of using embossment, an ultravioletsetting resin was used to form the uneven surface profiles of the filmbases corresponding to the fifth to ninth working examples. A urethaneresin system primer was applied on a polyethyleneterephtalate film witha thickness of 100 μm and was thermally cured to form a primer layerwith a thickness of 1 μm on the polyethyleneterephtalate film.

An ultraviolet setting resin solution with an epoxy acrylate system asthe primary component was coated on the corresponding rotating intaglioroll. Then, the composite film obtained above was pressed onto theintaglio roll with the ultraviolet setting solution and irradiated withultraviolet radiation to cure the resin solution. The solidifiedultraviolet setting resin is peeled off along with the film from therotating intaglio roll. A film base that serves as a prism layer or lenslayer made of cured ultraviolet setting resin on the primer layer isthereby created. This film base is used as the base for the opticalrotation selection layer in the second working example. By forming acholesteric liquid crystal layer on the surface of the resultant filmbase opposite to its uneven surface, the polarization light splittingfilm was completed.

FIFTEENTH WORKING EXAMPLE

A polarizing film with a thickness of 120 μm (made by Sumitomo ChemicalCo., Ltd., product name, “SUMILITE”) was adhered on the quarter-wavelayer of the second working example above (i.e., on the light emittingside). See FIG. 8B for the structure.

Evaluations

The transmissivity of polarized light, haze, and the luminance of thebacklight were measured using the following methods. The results arelisted in Tables 1 and 2.

(1) The polarized light transmission factor: the degree of thepolarization linearity in the linearly polarized beam obtained by eachof the polarization light splitting films of the above working examplesfirst to fifteen are evaluated in terms of the polarized lighttransmission factor. FIG. 14 shows the device arrangement used for themeasurement. An unpolarized light beam from a light source 51 istransformed into a parallel beam through a lens 52. The parallel beam isincident normally on a polarization light splitting film 53 with theoptical rotation selection layer side facing towards the light source 51and with the quarter-wave layer side facing in the opposite direction.To measure the linearly polarized component of the transmitted light, aGlen-Thomson prism 54 is rotated around the optical axis of the parallelbeam. Transmitted light is guided towards a photo-detector 56 through alens 55 and the intensity of the transmitted light is measured in twoconditions: when the polarization axes of the prism 54 and quarter-wavelayer in the polarization light splitting film 53 are perpendicular toeach other and when they are parallel to each other. The correspondingorthogonal transmission factor T⊥ (%) and the parallel transmissionfactor T∥ (%) were obtained for each of the above working examples.

(2) Haze: With a direct-reading haze meter made by Toyo Seki Co., Ltd.,haze is measured by placing the optical rotation selection layer side ofthe polarization light splitting film as the light receiving side andthe quarter-wave layer side as the light emitting side.

(3) Luminance: The luminance was measured for the liquid crystal deviceswith the structure shown in FIG. 10. A liquid crystal device with thepolarization light splitting film 10 of the fifteenth working examplearranged at the light receiving side of liquid crystal cell 30 wascompared with a liquid crystal device with a conventional polarizationplate. The luminance was measured in a direction normal to the displaysurface of the liquid crystal device using a luminance meter BM-7 byTopcon Corp.

TABLE 1 Measurement results for the polarized light transmission factorand haze Working Example T⊥ (%) T∥ (%) Haze (%) 1 10 75 5 2 10 80 3 3 565 30 4 5 82 10 5 5 80 10 6 5 80 10 7 5 80 10 8 5 80 10 9 5 80 10 10 583 5 11 5 83 5 12 5 83 5 13 5 83 5 14 5 83 5 15 0.5 35 3

TABLE 2 Measurement results for luminance Luminance in the normal(cd/cm²) Polarization light splitting film (Working Example 15) 85Standard polarizing plate 70

As shown in Tables 1 and 2, each embodiment had satisfactory results forthe polarization linearity and the haze value. The luminance wasimproved by approximately 20% (=(85−70)/70×100).

As described above, according to the present invention, the light usageefficiency of the backlight system can be improved with a relativelysimple structure. In addition, due to its relatively simple structure,the polarization light splitting film of the present invention hassuperior mass production capability. Also, it has a planar shape and issuited for designing thin backlight systems and thin liquid crystaldisplay devices.

In addition, by equipping the polarization light splitting filmaccording to the present invention with functional layers, such as adiffusion layer, prism layer, lens layer, and polarizing layer, itbecomes possible to assemble a backlight system having a simplestructure.

Furthermore, the backlight and liquid crystal device using thepolarization light splitting film of the present invention can easily bemade thin. Additionally, when a polarization light splitting filmincludes the functional layers, the system construction is simplifiedand complicated assembly processes are not necessary.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the polarization lightsplitting film of the present invention without departing from thespirit or scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A polarization light splitting film having a light receiving side and a light transmitting side, the polarization light splitting film comprising: an optical rotation selection layer at the light receiving side for reflecting one of right and left circularly polarized components of a light beam that is incident on the light receiving side and for transmitting the other one of the right and left circularly polarized components of the light beam; a quarter-wave layer laminated over the optical rotation selection layer at the light transmitting side and for emitting linearly polarized light; a linear polarizing layer laminated over the quarter-wave layer at the light transmitting side and having a polarization axis which substantially matches a polarization axis of the linearly polarized light emitted from the quarter-wave layer; and a diffusion layer disposed under the optical rotation selection layer at the light receiving side, the diffusion layer including a diffusion agent having an average particle diameter of 1 to 50 μm.
 2. The polarization light splitting film according to claim 1, further comprising a prism layer at the light receiving side for deflecting the incident light beam.
 3. The polarization light splitting film according to claim 1, further comprising a lens layer at the light receiving side for one of converging and diverging the incident light beam.
 4. The polarization light splitting film according to claim 1, wherein the optical rotation selection layer includes a base member and a cholesteric liquid crystal layer laminated on the base member.
 5. The polarization light splitting film according to claim 4, further comprising a diffusion layer laminated under the optical rotation selection layer at the light receiving side.
 6. The polarization light splitting film according to claim 5, further comprising a prism layer at the light receiving side for deflecting the incident light beam.
 7. The polarization light splitting film according to claim 5, further comprising a lens layer at the light receiving side for one of converging and diverging the incident light beam.
 8. The polarization light splitting film according to claim 4, further comprising a prism layer at the light receiving side for deflecting the incident light beam.
 9. The polarization light splitting film according to claim 4, further comprising a lens layer at the light receiving side for one of converging and diverging the incident light beam.
 10. The polarization light splitting film according to claim 1, further comprising a prism layer at the light receiving side for deflecting the incident light beam.
 11. The polarization light splitting film according to claim 1, further comprising a lens layer at the light receiving side for one of converging and diverging the incident light beam.
 12. The polarization light splitting fim according to claim 1, further comprising an anti-reflection layer at the light receiving side. 